Study Design. Analysis via computed tomography imaging software.
Objective. To determine parameters for ideal intralaminar screw trajectory and the feasibility of screw placement at L3, L4, and L5 laminae for pars defect fixation.
Summary of Background Data. To our knowledge, no studies provide anatomic parameters for ideal intralaminar screw trajectory for treating spondylolysis.
Methods. We used advanced imaging software for 3-dimensional interactive viewing to analyze 20 randomly selected normal adolescent lumbar computed tomographic scans. The ideal intralaminar screw trajectory was drawn from the inferior lamina, a point chosen to maximize cortical diameter at the isthmus of the lamina and bisect the pedicle. We measured and evaluated ideal trajectory parameters for percutaneous screw placement for pars defect fixation at the L3 to L5 laminae.
Results. The ideal pathway was the thick portion of the lamina between the inferior edge of the lamina and the pedicle. This area was close to the inferior articular process (axial plane), becoming more so at progressively caudal levels. At the ideal trajectory, the mean (standard deviation) coronal angle slightly decreased (L3–L5): 7.3° (5.1°), 6.6° (3.7°), and 4.2° (2.5°), respectively. The trajectory distance increased from cranial to caudal. These parameters increased (L3–L5): mean distance (transverse plane) between the starting point and middle of the spinous process, 1.2 (0.18 cm), 1.3 (0.2 cm), and 1.6 (0.3 cm), respectively; mean screw sagittal angle with respect to the posterior skin, 15.5° (5.0°), 24.3° (6.5°), and 43° (5.8°), respectively; and mean distance for guide wire entry, 28.8 (10.6 cm), 20.1 (5.4 cm), and 11.9 (2.1 cm), respectively.
Conclusion. At the ideal screw trajectory, pars fixation by intralaminar screw is uniformly feasible at L3 to L5 laminae, where most patients can accommodate a 4.5-mm screw.
Level of Evidence: 2
We used advanced computed tomography imaging software for the 3-dimensional interactive viewing and manipulation of 20 normal lumbar adolescent spines to define the anatomy and parameters for ideal screw trajectory at L3, L4, and L5 laminae for direct intralaminar screw fixation of pars defect.
*Department of Orthopaedic Surgery and the
†Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University, Baltimore, MD.
Address correspondence and reprint requests to Paul D. Sponseller, MD, MBA, c/o Elaine P. Henze, BJ, ELS, Medical Editor and Director, Editorial Services, Department of Orthopaedic Surgery, The Johns Hopkins University/Johns Hopkins Bayview Medical Center, 4940 Eastern Ave., A665, Baltimore, MD 21224-2780; E-mail: firstname.lastname@example.org
Acknowledgment date: May 6, 2013. Revision date: September 11, 2013. Acceptance date: October 14, 2013.
The manuscript submitted does not contain information about medical device(s)/drug(s).
No funds were received in support of this work.
Relevant financial activities outside the submitted work: consulting fee, support for travel to meetings funds.